In a Computed Radiography (CR) system, the laser beam is scanned over the surface of a storage phosphor screen by a galvanometer in the fast scan direction while the screen is transported under the scanline in the slow scan direction. To make the CR reader compact and manufacturable, a final fold mirror is placed in the optical path between the F-theta lens and the phosphor screen. The fold mirror introduces three degrees of freedom into the optical system. A coordinate system on the mirror has an x-axis along the fast scan dimension of the mirror, a y-axis in the plane of the mirror perpendicular to the x-axis and a z-axis normal to the mirror surface.
The z-axis degree of freedom allows the path length of the optical system to be adjusted by translating the mirror along that axis. Typically, the depth of focus of the laser is not critical and there is no need to adjust this degree of freedom.
The other two degrees of freedom are rotations about the x and y axes which allow the scanline to be positioned on the phosphor surface. It is required to rotate the scanline perpendicular to the slow scan transport direction to eliminate a parallelogram image distortion. It is also required to translate the scanline to a particular position where the laser beam is not obstructed and the phosphor screen is well controlled for height. Adjusting the scanline to the correct angle and position is practically achieved by having a sensor at each end of the scanline. These sensors are implemented behind slits along the scanline. When the scanline hits these sensors the correct angle and position is achieved.
The final fold mirror adjustment mechanism in prior CR readers used two orthogonal rotation axes. The first axis was always along the x-axis of the mirror, which provided the translation of the scanline. The second axis was perpendicular to the first axis, sometimes oriented vertically, which provided rotation of the scanline. However this axis had the problem of also translating the scanline. This coupling of the two adjustments means that an iterative adjustment process is needed.
The first axis is adjusted until sensor 1 turns on. Then a search is made using the first axis to determine whether the scanline is ahead of or behind sensor 2. The first axis is readjusted to turn sensor 1 on and the knowledge gained during the search is used to turn axis 2 in the appropriate direction to correct the scanline rotation error. However as the adjustment is made with axis 2, sensor 1 will turn off because axis 2 translates as well as rotates the scanline. It is not easy to determine that the correct rotation has been achieved. Axis 1 is adjusted again to turn on sensor 1, and sensor 2 is observed. It may or may not be on. This back and forth between the two axes is continued until both sensors are turned on simultaneously. Thus a time-consuming iterative process is used to adjust the final fold mirror.